Gold refinement is the industrial process of separating pure gold from other metals, such as silver, copper, and platinum group metals, to achieve high purity levels. This intensive purification is necessary because the raw gold material, known as dore, is rarely more than 90% pure after initial extraction and smelting. Refinement transforms this semi-pure alloy into a product that meets the stringent quality standards required for investment, electronics, and specialized industrial applications.
Preparing the Raw Material
Refinement begins with dore bars, sourced from mining operations or recycled scrap gold. Dore is a rough alloy typically containing 50% to 90% gold, along with silver and base metals like copper, zinc, and lead. Before specialized refinement, these bars undergo preliminary preparation to remove the bulk of non-precious metals.
The initial step involves smelting, where the dore is melted in a furnace at high temperatures. Chemical agents called fluxes, commonly borax and soda ash, are added to the molten metal. These fluxes react with the base metals and impurities, forming a low-density liquid layer called slag that floats on the surface of the heavier, molten gold alloy. The slag is skimmed off, consolidating the gold into a higher-concentration alloy ready for the next stage.
Chemical Refinement Using the Miller Process
The first major purification technique often employed is the Miller Process, a rapid chemical procedure. This method involves bubbling chlorine gas directly through the molten gold alloy held in a furnace. The chlorine reacts selectively with impurities, such as silver and base metals, because they have a greater chemical affinity for chlorine than gold does at high temperatures.
These reactions form molten metal chlorides, which are insoluble in the liquid gold and rise to the surface as a separate layer of slag. This chlorine-based process is valued for its speed and cost-effectiveness, quickly producing gold with a purity of approximately 99.5% fine. However, this level of purity is not high enough for applications requiring ultra-pure gold, necessitating a further refinement step.
Electrolytic Refinement Using the Wohlwill Process
To achieve the highest possible purities, the gold from the Miller Process is subjected to the Wohlwill Process, an advanced electrolytic technique. This method uses the impure gold, now about 99.5% pure, to cast anodes which are submerged in an electrolyte bath. The electrolyte is a solution of gold chloride and hydrochloric acid, and the bath also contains thin sheets of pure gold that serve as cathodes.
When an electric current is applied, the gold in the impure anode dissolves into the electrolyte as gold ions. The electric field drives these positively charged gold ions to the negatively charged cathode, where they deposit as solid, ultra-pure gold. Most of the remaining impurities, including platinum group metals, either stay in the electrolyte solution or fall to the bottom of the tank as a valuable sludge known as anode slime. This precise electrochemical separation allows the Wohlwill process to yield gold with purities of 99.99% and even up to 99.999% fine.
Final Products and Purity Standards
Once the Wohlwill process is complete, the highly pure gold deposited on the cathodes is removed, melted, and cast into standardized forms. The most common final products are investment-grade bullion bars, wafers, and grain used for manufacturing. Valuable byproducts, such as silver and platinum group metals, are recovered from the Miller Process slag and the Wohlwill anode slime and processed separately.
Purity is measured using fineness, which expresses the gold content in parts per thousand. The industry standard for investment-grade bullion, such as that specified by the London Bullion Market Association (LBMA) Good Delivery rules, requires a minimum fineness of 995.0 (99.5% pure). Higher purities are referred to by the number of nines: “four nines” (999.9) is standard for retail bullion, and “five nines” (99.999%) is reserved for specialized applications. This ultra-high purity gold is necessary for high-tech industries where trace elements could compromise performance.